When a material, particularly a thermoplastic material, is formed, it flows in a ‘fluid-viscous’ state inside a machine and it is then cooled, taking its final and desidered form. The knowledge of our transformation machines and of the variables linked to this machine is fundamentally based on the practice experience of the operators on the machines, on the type of the machines and on the designer of the machine.
All these experiences is grounded on the behavior of the material during the processing, on the temperature, stress and pressure’s profiles inside the processing machine.
Trying to understand the behavior of the material subjected to these conditions is the target of rheological measurements.
RDLAB137 can help you in seting up the best methods for measuring viscosity and rheological behaviour of the materials in conditions of interest, by rotational rheometers, capillary rheometers and with specific studies.
RHEOLOGY is the Science which study the properties of flux and deformation of the Matter. Generally speaking, regarding pure polymers or additivated plastics, the rheology results to be very complex and the rheological behviours are distant from ideal behaviors and they depend on many variables, as shear rate, molecular weight, structure of the polymeric chain, type and concentration of additives, temperatures, pressures, times of residence and so on.
Rheological study is of fundamental importance for macromolecular based materials for 2 main reasons:
- During the production of a plastic material, the rheology of the melt and of the grains determinates the stresses during all the processing operations, as extrusion, calandering, blowing, spinning, coating, moulding etc.
- The rheological properties have important consequences on the final mechanical behavior of products, as residual stresses in the materials, particularly when orientation is important (fibers, foamed materials and so on)
Finally, the rheology of grains or dusts has great importance too, for example in the mixing and weighing phases of processing.
In order to understand which is the best instrument or measurement for our purposes, first thing to do is to consider which are the conditions of or transformation process, and particularly the speed of deformation on which our materials are subjected.
Sample preparation and conditioning
Differently from mechanical testing, rheological measurements do not need particular preparation of samples. However care must be take in order to avoid contamination of the samples, for example by water: a sample with moisture can give lower viscosities than expected. It is suggested a conditioning at controlled temperature and humidity.
In a capillary viscometer, the material, at definite and controlled temperature, in a fluid-viscous state is forced into a capillary, which is generally a empty cylinder with fixed diameter and length.
Speed of flow, force on the material and pressure before the capillary are measured continuatively; results are rheological curves in which generally is plotted the viscosity as a function of the rate of shear speed (shear rate).
Instruments works normally at high temperatures (up to 400°C) and have different mechanical parts in movement: a good practice is needed and only with experience you could achieve good and reproducible data. The cleaning of the instrumentation is a very important factor and can take a lot of time.
Rheological measurements are full of informations, but, they need to be analyzed and interpreted.
This is an example of rheological measurements on 2 materials:
Material “A” shows higher viscosities at lower shear rates, and lower viscosities at higher shear rates, if compared with “B” material. In this case, “A” material show a lower MFR than “B” but, during the process, the behavior can be opposite: MFR is a rheological measurement too, but it is measured only at very low shear rates and can be misleading.
Different instruments are available on the market: generally speaking the torsion stress is measured and the viscosity curves are measured as a function of shear rate. Adding to flux curves, instruments can measure the viscous and elastic parts of the material (viscoelasticity).
Traditionally, rotational viscometers are used for low shear rate, till to almost zero sec-1 and generally the can not reach the higher shear rates of capillary rheometers. For a really complete rheological characterization, the best should be to utilize both type of instryumentation.
With actual data elaboration, you do not need calculation fro obtaining the data. Great care must be taken to cleaning of the mechanical parts of the instrument, and a specific experience is important in order to consider all the variables linked to the material (air, moisture, etc.).
Behavior of the material can be very strange, and sometimes you have to analyze materials that really ‘change’ the rheological properties with time.
Viscosity of a material can be measured directly on processing or pilot machines. These possibilities are specific and sometimes instruments are designed ad hoc. This type of measurement are the nearest to processing conditions; however, on the other side, measurements are sometimes very difficult and are not so ‘universal’ as standard laboratory measurements.
DMTA=Dynamic Mechanical thermal Analyzer. These measurements combine rheological measurements with thermal measurements and can give many interesting information, but you need to understand all the details of the test. As a function of temperature, in short times, you can have information on real and complex viscosity, with different clamping geometries and frequency of sollecitation.
Sample preparation and conditioning
This phase need an adequate attention, particularly if samples are solid. Same consideration for mechanical testing are valid.
Instrumentation work on small samples and tests can be carried out in a wide temperature range (form liquid nitrogen to 500°C or more).
Sample positioning and clamping is fundamental. Theory and experience are needed for measure data and analyze results.
Variables are many: frequency, stress, amplitude of sollecitation, elastic and loss forces, and so on. Befor to buy such a type of instrumentation, we must be prepared to control and understand all these variables on our materials.
SPECIFIC VISCOSITY MEASUREMENT
Measure of viscosity can be don with a lot of other, sometime more specific, instruments.
Brookfield, Severs, Grader (MFR) are only examples of specific apparatus for viscosity measurement.
For very specific needs, it is possible to design and realize adequate instrumentation.
Dr. Maurizio Veronelli – RDLAB137 srl
Specialist in Polymer Science